The use of dispersed alkali metal borates in lubricant formulations is well known. The patent literature has taught the combination of an alkali metal borate with sulfur compounds and particular phosphorus compounds. See for example, U.S. Pat. Nos. 4,717,490; 4,472,288; and patents cited therein. However, these prior art formulations suffer from shortened shelf life compared to other commercially available lubricants which do not use solid dispersions of borate. The phosphorus chemistry taught in the prior art relies on acidic compounds, which were needed for improvements in load-carrying ability and protection against seal leaks in the presence of water.
U.S. Pat. No. 4,717,490 to Salentine discloses a lubricating composition that is a combination of alkali metal borates, sulfur compounds, dialkyl hydrogen phosphite, and a mixture of >50% neutralized acidic phosphates. However, this composition suffers from a shortened shelf life compared to other commercially available lubricants, which do not use solid dispersions of alkali metal borates. In particular, this composition will exhibit additive “dropout” over time. The problem becomes more severe as the storage temperature increases. The standard remedy in the industry is to add more dispersant or detergent additives to the composition to improve the shelf life. However, these additives can negatively impact other performance properties of the gear lubricant. It is, therefore, an object of the present invention to provide an alkali metal borate-containing lubricant which has superior load carrying properties and improved storage stability.
Without being bound by a specific theory, we have discovered a major cause of the shortened shelf life for borate-containing formulations. The acidic phosphorus compounds, those with a hydrogen attached directly to a phosphorus or attached to a heteroatom which is in turn attached to a phosphorus, which were previously relied on for other performance benefits, appear to react with either the borate particles or with the basic dispersant and detergent additives that are used to stabilize the borate particles and to form a precipitate which settles to the bottom of the lubricant container. Although U.S. Pat. No. 4,717,490 to Salentine refers to the use of neutralized phosphates, the phosphates are only partially neutralized. In addition, U.S. Pat. No. 4,717,490 specifies use of a dihydrocarbyl phosphate, which contains an acidic hydrogen. We have found that using only non-acidic phosphorus compounds will result in much better shelf life without sacrificing either the load-carrying or seal-leak protection properties of the gear lubricant. In addition, load-carrying ability can be improved by selection of appropriate ratios of polysulfides.